Reflector communications channel for automatic protection switching

ABSTRACT

An apparatus and method for a communications network including at least one interface circuit reads frame data received from the communications network and writes frame data to be transmitted over the communications network, the frame data including a plurality of transport overhead fields. The apparatus includes signature logic coupled to the interface circuit, the signature logic identifying signature data and writing the signature data into transport overhead fields in an outgoing frame. Reflector logic coupled to the interface circuit copies data from one of the received transport overhead fields, the copied data being placed into a transport overhead field in the outgoing frame, the copied data including the received signature data. The interface circuit compares the copied data to earlier received frame data from the communications network, the determination of a mismatch identifying a transition requiring an update of at least one routing table. The data is used to determine configuration compatibility between interfaces and among a plurality of tributary interfaces and to eliminate dependence on multiplexers that transmit to routers with protect circuits according to protection switching protocols. A method for a communications network includes transmitting signature data in a transport overhead field, the data identifying one of interfaces in the local routers, returning the data to the local router, and configuring a communications relationship using the data. The method includes using the data to determine which is an active interface and to determine whether to update at least one routing table and using the data to configure the interface circuits.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to network communications, and morespecifically, automatic protection switching in network communications.

2. Description of the Related Art

A collection of interconnected networks is an “internet.” An internet isformed when separate networks are connected together. The specificworldwide computer internet, the “Internet” refers to a global computernetwork that consists of internetworked communications systems. Computernetworks such as the Internet use a protocol hierarchy organized as aseries of layers. The combination of layers and protocols for a givencomputer network is referred to as a network architecture. Each layermay have a protocol governing the method of communication for the layer.A protocol refers to rules that govern the format of frames and packetsthat are exchanged via a computer network to a peer entity. One suchprotocol, “IP” (Internet Protocol) defines the packet format for anetwork layer.

Another layer of a computer network is the physical layer. The physicallayer relates to the transmitting over a communication channel andconcerns the physical transmission medium. One physical medium used forcomputer networks today is optical fiber. The optical fiber standardused for most long-distance telephone connections is Synchronous OpticalNETwork (SONET).

A similar standard to SONET is the Synchronous Digital Hierarchy (SDH)which is the optical fiber standard predominantly used in Europe. Thereare only minor differences between the two standards. Accordingly,hereinafter any reference to the term SONET refers to both SDH and SONETnetworks, unless otherwise noted.

SONET provides a standard for multiplexing multiple digital channelstogether. SONET is a time division multiplexing system that devotes theentire bandwidth of the fiber to one channel that contains time slotsfor various subchannels. Accordingly, it is a “synchronous” system. ASONET line transmits digital information in bits at precise intervals.SONET systems are made up of switches, multiplexers and repeaters. InSONET terms, the transmission between two endpoints can be conceptuallybroken into a hierarchy of sub-parts, comprising of a “section,” thefiber connection directly connecting one device to another device; a“line,” the fiber connection between any two network elements thatterminate lines, i.e., line terminating equipment (LTEs) includingmultiplexers; and a “path” the fiber connection between a source and adestination that terminates in path terminating equipment (PTEs). A lineconsists of several sections, and a path consists of several lines.Topologically, a SONET system can be either a mesh or a dual ring. Notethat the term “line” used in a SONET network is equivalent to the term“section” in a SDH network.

Data transmitted according to the SONET standard is in frames, which,according to one standard, are organized as blocks of data with a blockof 810 bytes transmitted every 125 μsec. The number of bytes transmittedper frame varies with the transmission rate of the connection, but inall cases frames are transmitted at a rate of 8000 frames per secondwhich matches the sampling rate of pulse code modulation channels thatare used in digital telephony systems.

The SONET system specification includes provisions for automaticprotection and reconfiguration in case of failure, called APS (AutomaticProtection Switching) in the SONET specification, and MSP (MultiplexSection Protection) in the SDH specification. An APS/MSP configurationcould include one ‘protect’ interface circuit and one ‘working’interface circuit. One such APS configuration is known as “1+1 linearAPS”. In a 1+1 linear APS configuration, any data transmitted by anetwork element is transmitted to both the working circuit and theprotect circuit connected to the network element.

When routers are configured as network elements in a SONET systemconfigured for APS, the interface circuits connecting the routers to theworking and protect circuits may be configured to be located in separaterouters or the same router. Protection occurs at the SONET line level(in SDH terminology, protection occurs at the SDH section level).Protection control bytes transmitted between LTEs, the end points of aSONET line, communicate APS protection information. This protectioninformation includes whether the protect or working circuit is currentlyactive. Normally, when all equipment is functioning correctly, trafficis carried by the working circuit. In APS configurations involvingrouters, the working interface is active and the protect interface isinactive. If the working circuit fails, an “APS switch” occurs, causingdeactivation of the working circuit and activation of the protectcircuit. In APS configurations involving routers, the working interfaceis deactivated and the protect interface is activated.

The transmissions between LTE are synchronized by the protectioninformation transmitted in the protection control bytes. Some SONET/SDHmultiplexers do not comply with the SONET/SDH standards, and do notappropriately transmit protection control bytes to the other end of theline segment. Such cases preclude implementation of protection logic bythe local router.

What is needed is a system capable of implementing protection logic onSONET/SDH lines connecting routers to multiplexers that do not transmitprotection control bytes on the SONET/SDH line.

SUMMARY OF THE INVENTION

Accordingly, an apparatus, system and method for a communicationsnetwork is provided that implements protection logic on SONET/SDH linesconnecting routers and multiplexers without using multiplexers complyingwith SONET/SDH specifications. The system and apparatus includes atleast one interface circuit that reads frame data received from thecommunications network and writes frame data to be transmitted over thecommunications network, the frame data including a plurality oftransport overhead fields. The system and apparatus further includessignature logic coupled to the interface circuit, wherein the signaturelogic determines and writes the signature data into transport overheadfields in an outgoing frame. The system and apparatus also includesreflector logic coupled to the interface circuit, wherein the reflectorlogic copies signature data from at least one of the received transportoverhead fields, the copied data being placed into a transport overheadfield in the outgoing frame, the copied data including the receivedsignature data. The identifying signature data includes data identifyingthe interface as one of an automatic protection switching (APS) workingcircuit, an APS protect circuit, and a non-APS circuit. Additionally,the interface circuit compares the copied data to earlier receivedsignature data from the communications network to determine whether thecopied data matches the earlier received signature data, thedetermination of a mismatch identifying a route transition requiring anupdate at least one routing table.

According to another embodiment, the reflected signature data receivedby an interface is used to determine configuration compatibility betweenthe interface and a second interface, and between the two interfaces anda plurality of tributary interfaces.

An embodiment related to a method for a communications network includesa plurality of interfaces in at least one local router and at least oneremote router, wherein the method includes transmitting signature datain a transport overhead field to at least one remote router, the dataidentifying one of the interfaces in the local routers. The methodfurther includes the step of returning the data to the pluralityinterfaces, and, in the local router, configuring a communicationsrelationship using the returned signature data. The method also includesusing the signature data to determine whether a multiplexer transitionedfrom reading one of the plurality of interfaces to another of theplurality of interfaces. According to an embodiment of the method, thedata is transported via a transport overhead field which is a path leveloverhead field of a frame, such as a path trace byte. The path leveloverhead field is received and transmitted by a plurality ofintermediate add-drop multiplexers, the plurality of intermediateadd-drop multiplexers maintaining the transport overhead field.

According to another embodiment, the method includes comparing thesignature data to earlier received signature data from thecommunications network to determine whether to update at least onerouting table and using the data to determine configurationcompatibility among a plurality of interface circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a block representation of a communication system includingSONET/SDH networks.

FIG. 2 is a generalized block diagram of a SONET/SDH networkimplementing a techniques in accordance with embodiments of the presentinvention.

FIG. 3 is a block diagram of a SONET/SDH network without AutomaticProtection Switching/Multiplex Section Protection capable ofimplementing a reflection technique in accordance with an embodiment ofthe present invention.

FIG. 4 is a block diagram of a SONET/SDH network demonstrating the useof a reflection technique during signaling link failures between aworking interface in a router and a protect interface in a router inaccordance with an embodiment of the present invention.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1, block representation of a Synchronous DigitalHierarchy (SDH)/Synchronous Optical NETwork (SONET) communication system100 shows a communications network including fiber optic networks. Thenetwork includes a United States backbone network 110 and a Europeanbackbone 130 networked together with a regional-level network 120 and aEuropean national-level network 140. Networks 120 and 140 include “WideArea Networks” (WANs) that also include fiber optic networks. Attachedto the regional-level networks 120 could be “Local Area Networks”(LANs). The communication system 100 includes networks following theSONET or the SDH protocols for transmitting data organized into SONET orSDH frames.

Referring to Table 1, a portion of a SONET frame, 9 rows and 270columns, shows the organization of a SONET OC-3c (concatenated) frame,according to the BellCore STS-3c standard for SONET, which isincorporated herein by reference. The frames are transmitted row by row,from top to bottom, column byte by byte, 8000 frames/sec.

TABLE I

NOTE: (Section Overhead (SOH) (3 × 9 Bytes), + Pointer (1 × 9 Bytes), +Line Overhead (LOH) (5 × 9 Bytes), + Path Overhead (POH) (9 × 1 Bytes),)= Transport Overhead for OC-3c (STS-3c) Frames. The POH is the 10th Bytein each of the 9 rows (J1, B3, C2, G1, F2, H4, Z3, Z4, Z5).

The OC-3c designation indicates that the carrier is not multiplexed, butcarries data from a single source. Thus, the data stream is from asingle source at 155.52 Mbps with three OC-1 streams within an OC-3cstream interleaved by column. The interleaving of streams produces aframe 270 columns wide and 9 rows deep. An OC-3c stream produces moreactual user data than an OC-3 stream due to the path overhead columnbeing included inside an SPE once instead of three times as is the casefor three independent OC-1 streams. Accordingly, as shown in TABLE 1,260 of the 270 columns within the frame are available for user data inOC-3c as compared to 258 columns available in OC-3. A similar protocolto OC-3c is provided for European systems in ITU G.783. Although theOC-3 standard is presented, one of ordinary skill in the art with thebenefit of the disclosure herein appreciates that the embodiments hereindescribed apply to other SONET and SDH standards.

As shown in TABLE 1, the first ten bytes of a SONET OC-3c frameconstitute transport overhead, followed by 260 bytes of SynchronousPayload Envelope. The ten bytes of transport overhead include sectionoverhead, line overhead and path overhead bytes. In general, certainbytes of the transport overhead only travel between each section, andare reconfigured at each section boundary. Other transport overheadbytes travel through section boundaries and are reconfigured at lineboundaries. For example, an Add-Drop Multiplexer (ADM) receiving a SONETframe will interpret the line overhead bytes. These line overhead byteswill not be sent on through the network. Instead the ADM generates newline overhead bytes for transport through the network. Unlike the lineoverhead and section overhead bytes, path overhead bytes are receivedand interpreted at the ends of a path. Accordingly, a frame received byan ADM will neither remove nor reconfigure path overhead bytes.

Referring now to FIG. 2, a SONET/SDH network is shown implementing a 1+1linear APS system, in each of two locations, one local and one remote.In both locations, the line overhead bytes are received and regeneratedanew by the line terminating equipment (LTE) at the ends of SONET/SDHlines. As shown, a first Add-Drop Multiplexer (ADM) 210 is connected viaone or more SONET/SDH fiber optic lines to a second ADM 212. Each of ADM210 and ADM 212 are coupled to at least one router via a SONET/SDH fiberoptic line 205. For example, ADM 210 is shown coupled via SONET/SDHfiber optic line 205 to working interface 214 housed in router 202; ADM210 is also coupled via another fiber optic line 205 to and protectinterface 216 housed in router 201. Working interface 214 and protectinterface 216 optionally may be housed in the same router. ADM 212 isshown coupled via SONET/SDH fiber optic line 205 to remote workinginterface 218; ADM 212 is also coupled via another fiber optic line 205to remote protect interface 220. Both remote working interface 218 andremote protect interface 220 are housed in router 209. Each of theinterfaces, 214, 216, 218 and 220 are SONET/SDH path terminatingequipment (PTE) interfaces.

Normally, in 1+1 linear APS, ADM 210 bridges all data to be transmittedto the working and protect interface circuits 214 and 216 via the fiberoptic lines 205.

In system terminology, the connection between router 202 and ADM 210 isa line level connection. The connection between router 201 and router209 is a path level connection. The connection between an ADM such asADM 210 and any other elements in a SONET network consists of one ormore line level connections.

A routing protocol running in each router, such as the higher-levelrouting protocols “Intermediate System to Intermediate System” (IS—IS),or “Border Gateway Protocol” (BGP), based on the Internet Protocol (IP),maintains one or more routing tables. The routing tables associateoutgoing interfaces with destination network addresses. Ideally, when anetwork configuration changes, as it does because of an APS switch,every involved router receives immediate notification of the newconfiguration.

Referring now to TABLE 1 and FIG. 2 in combination, the “line” overheadbytes for a SONET system include bytes designated “K1” and “K2”. Thesebytes provide a communication channel for carrying information relatedto Automatic Protection Switching (APS). Byte K2 is also used to carryline layer maintenance signals.

According to the SONET/SDH specifications for a 1+1 linear APS or MSPsystem, any data transmitted to ADM 210 from ADM 212 is transmitted toworking interface 214 housed in router 202 and to the protect interface216 also housed in router 201. Likewise, ADM 212 transmits data to theremote working interface 218, and to remote protect interface 220, bothhoused in router 209. A SONET network implementing APS uses bytes K1 andK2 in the line overhead portion of the frame to identify the interface,either working or protect, from which an ADM is currently receivingdata. Thus, for example, ADM 210 bridges all transmissions equally totwo separate interfaces, working interface 214 and protect interface216, but “listens” to only one of the two interfaces. The working andprotect interfaces 214 and 216 are managed by logic running in therouter containing the protect interface, such as router 201, whichcontains protect interface 216. The APS logic activates for transmittingonly the interface currently selected for “listening” by the ADM. Thenon-selected interface is held in a “Protocol Down” state by the routerto prevent transmission of data packets on the circuit. For example, ifADM 210 selects the working interface 214, the APS logic in router 201commands router 202 to activate the working interface 214. Regardless ofwhether a protect interface or a working interface is currentlyselected, the protect interface 216 conducts an ongoing protocol dialogwith ADM 210, using SONET bytes K1 and K2 or appropriate SDH bytes inthe line overhead.

Normally, a protect interface depends on the APS or MSP protocoltransmitted by the ADM to determine whether the ADM is currentlyselecting the working or protect circuit. Lacking this determination,the APS logic running in the router containing the protect interface isunable to determine whether to activate the working interface or theprotect interface.

For example, assume ADM 210 does not transmit APS/MSP protocol. ADM 210detects errors on the tributary circuits, and selects either the workingor protect circuit. ADM 210 thus transmits all data to both workinginterface 214 in router 202 and protect interface 216 in router 201.However, because ADM 210 does not transmit APS/MSP protocol, the APSlogic in router 201 is unable to determine whether it should activatethe working interface 214 or the protect interface 216. Therefore,protect interface 216 is unable to respond appropriately to an APSswitch in ADM 210. As a result, an APS switch in ADM 210 results in amismatch between the selection state of the ADM and the selection stateof the router interfaces. Further, the currently active interface doesnot correspond to the circuit currently selected by the ADM. Thus,communication is lost between the currently active interface, either theworking interface 214 or the protect interface 216, and the currentlyactive interface connected to ADM 212, either remote working interface218 or remote protect interface 220.

Systems configured for APS/MSP currently do not provide earlynotification of an APS switch to the currently selected remoteinterface. For example, assume ADM 210 is configured to receive APSbytes from protect interface 216. If a break in the fiber optic linebetween the working interface 214 and ADM 210 occurs, an APS switch willtake place within 50 ms., resulting in the activation of the protectinterface 216, and the deactivation of the working interface 214. Suchan APS switch necessitates a routing reconvergence before data flow isrestored.

The BellCore specification GR-253, incorporated herein by reference,requires APS switches to be completed within 50 ms of a failure causingan APS switch. Although an APS switch may be complete within 50 ms, theswitch necessitates rewriting of routing tables in all routers affectedby the APS switch before data flow is restored. For example, assume theSONET/SDH system shown in FIG. 2 is a network following an InternetProtocol (IP). Should ADM 210, working interface 214 and protectinterface 216 require a state transition due to an APS switch, therouting tables must be updated in at least the currently active memberof the pair of routers 201 and router 202, and router 209 connected toADM 212, to enable forwarding of data to the currently active interfaceattached to ADM 210. In system terminology, the connection between thetwo directly-communicating routers, the “adjacency” of the routers, mustbe established before traffic can flow between the two routers.

Prior to the APS switch that causes interface deactivation in workinginterface 214 and the activation of the protect interface 216, anadjacency exists between working router 214 and the currently activerouter connected to ADM 212. This adjacency is reflected by adjacencyentries in the routing tables of routers 201 and the currently-activerouter connected to ADM 212. The adjacency must be deleted from therouting table of the current-active router connected to ADM 212 before anew route can be established, leading from that router to thenewly-activated protect interface 216. Typically, a route reconvergencecan require a lengthy time-out, much longer than the 50 ms APS switchingtime before communication is reestablished.

The system and method of the present invention solves the problemsdiscussed above in both SONET and SDH systems. The system and methodestablishes a communication channel between a remote router and theabove-described APS logic running in a router containing a protectinterface. According to an embodiment of the system and method, aportion of a transmitted frame dedicated to “path” level communicationprovides a communication channel for carrying information concerningwhether a working or protect tributary is currently selected by the ADM.Further, according to an embodiment, all router interfaces writedistinctive identifying signature data, the signature data identifying asending router interface as a working interface, a protect interface ora non-APS interface.

More specifically, referring to FIG. 2, according to an embodiment ofthe present invention, working interface 214 and protect interface 216each transmits distinctive identifying signature data in the form ofbits in a path overhead byte, as in, for example, a path overhead bytein a SONET OC-3c (concatenated) frame or a similar path overhead byte inan SDH frame. ADM 210 selects one of the circuits 205 as the activecircuit, either the circuit connecting ADM 210 to working interface 214,or the circuit connecting ADM 210 to protect interface 216. Thesignature data is transmitted to ADM 212 and transmitted to both remoteworking interface 218 and remote protect interface 220. Router 209determines, from the received bits in the path overhead byte holding thesignature data, whether ADM 210 is transmitting data frames from workinginterface 214 or protect interface 216 to ADM 212. One of skill in theart with the benefit of the information contained herein will appreciatethat the disclosed system does not depend on MPS or APS switchinginformation generally transmitted in line overhead bytes in a SONETframe or SDH frame. The time required for a router at one end of a pathlevel connection to detect a transition by an ADM affecting a router atanother end of a path level connection is greatly reduced.

More specifically, router 209, using control logic, actively keeps trackof which of working interface 214 and protect interface 216 is currentlyselected by ADM 210. Thus, if working interface 214 fails and an APSswitch takes place in ADM 210, the path overhead bytes received byremote router 209 would contain identifying signature bits of protectinterface 216. When the value of incoming identifying signature datachanges, router 209 recognizes that an APS switch of interfaces hasoccurred at ADM 210. Then, router 209 notifies the associated routingalgorithm in router 209 that the now-defunct adjacency with router 202should be deleted immediately, without waiting for the normal time-outto expire.

The above method advantageously provides advance notification to remoterouters of an APS/MSP switch, thereby decreasing the time required tocomplete the reconfiguration of routing tables made necessary by theAPS/MSP switch. The advance notification occurs independently of whetherthe remote routers are configured for APS/MSP, or whether the ADMsinvolved comply with the requirement to communicate with a protectinterface using the APS protocol. A remote router uses the incomingvalues of the identifying signature to actively keep track of which APSinterface is currently selected by an ADM. When the value of an incomingidentifying signature changes, the remote router recognizes that an APSswitch has occurred. If a switch is detected, the remote router willnotify the routing algorithm in the remote router that an adjacency witha deselected router should be reconfigured.

According to an embodiment of the present invention, the path overheadportion of every frame received by a remote router includes theidentifying signature of the interface that transmitted the frame. In aSONET/SDH system, the information identifying a source interface as aworking interface, a protect interface, or a non-APS interface, isoptionally located in a path level overhead field. Referring back toTable 1, one such path overhead field is the path trace message,conveyed by the path trace byte J1 byte in a SONET system. Using the J1byte, a local PTE injects identifying signature data, such as a bitstring, into a frame for transmission, wherein the bit string identifiesthe transmitting router and PTE. One of ordinary skill in the art withthe benefit of this disclosure will appreciate that other path overheadfields are capable of carrying the identifying signature and such otherfields are within the scope of the present invention.

According to another embodiment, referring to FIG. 2, in a 1:n linearAPS system in which data is not bridged to both router 202 and router201, identifying signature data is copied into the path overhead ofevery frame transmitted by the remote router. The path overhead ispassed through to either working or protect interfaces within thereceiving router because path overhead bytes are received andinterpreted at the ends of a path according to both SONET and SDHspecifications. The remote receiving router detects the signature datainjected into the frame and interprets the detected signature data todetermine whether the transmitting interface, whether working orprotect, has changed since the last detected signature data. Thedetermination provides early notification to the receiving interface ofwhether an APS switch occurred at another end of the path. One ofordinary skill in the art appreciates that this early notification isappropriate for at least 1+1 linear APS and 1:n linear APS systems.

Using the signature data, a router at one end of a SONET path is able todetermine when an APS switch has occurred at another end of the path.Additionally, the reflected signature data enables a router to implementprotection logic even if an ADM to which the router is directlyconnected does not transmit protection control bytes.

Another embodiment of the present invention includes an apparatus for acommunications network, such as a SONET or an SDH communication network.The apparatus includes at least one interface circuit, the interfacecircuit located, for example, in a remote router.

The apparatus further includes signature logic coupled to the interfacecircuit, wherein the signature logic places identifying signature datainto a path overhead field of a frame. The signature data includesinformation identifying the interface circuit as an APS workinginterface, an APS protect interface, or a non-APS interface. Accordingto one embodiment, the path overhead field is a path trace byte, whichis present in both SONET and SDH systems. In successive frames, the pathtrace byte transmits the contents of a multi-byte path-trace message.The length of the message is configurable as 64 bytes or 16 bytes.Depending on the configuration, therefore, the entire contents of a pathtrace message is transmitted every 64 or 16 frames. One of ordinaryskill in the art will appreciate that other path level fields inSONET/SDH are capable of transmitting the copied data and are within thescope of the present invention.

Also included in the apparatus is reflector logic coupled to theinterface circuit, wherein the reflector logic copies signature datafrom at least one of the path overhead fields in an incoming frame andwrites the signature data into a path overhead field in an outgoingframe. The signature data in the outgoing path overhead field includesinformation identifying the original sender of the incoming pathoverhead field. The identifying signature data includes data identifyingthe originating interface as either an APS working interface, an APSprotect interface, or a non-APS interface.

Also included in the apparatus is reflector logic coupled to theinterface circuit, wherein the reflector logic copies the signature datafrom one of the received path overhead fields. The reflector logic maybe coupled to the interface circuit, both of which may be located in aremote router.

The copied signature data is placed into a transport overhead field inthe outgoing frame. The copied signature data includes the receivedidentifying signature data. The outgoing frame may include either aSONET or SDH frame for transmitting over the communications network. Thetransport overhead field includes any field capable of being transmittedas a “path” level field. The copied data in the transport overhead fieldincludes information identifying a source interface. For example, thecopied data includes data received by the remote router from a localrouter.

According to this embodiment, the copied identifying signature data istransmitted via path overhead bytes for retransmission to an originatinginterface. The copied identifying signature data is detected as havingbeen transmitted by a remote interface. Using the copied identifyingsignature data, the receiving interface determines whether the originaltransmitting interface matches the identifying signature data of thereceiving interface. If the identifying signature data does not matchthe identifying signature data of the receiving interface, a change isthereby detected, permitting the interface to detect when an APS switchhas occurred in a connected multiplexer. The connected multiplexer,therefore, is not required to transmit protection control bytes for theAPS switch to be detected by the receiving interface.

According to an embodiment, the apparatus further includes another oneor two interface circuits coupled via the communications network to theinterface circuit. The one or two interface circuits are capable ofreceiving the copied signature data. For example, a local router thatinitially transmitted the data in the path overhead field to the remoterouter includes an interface circuit capable of receiving the copieddata. Likewise, a router with protect interface is also capable ofreceiving the copied signature data. In systems in which an ADMtransmits data to both attached routers, such as in 1+1 linear APSsystems, as well as in systems in which an ADM transmits data to justthe active router, such as in 1:n linear APS systems, the router holdingthe active interface attached to such an ADM will receive the reflecteddata.

The ADM receiving the signature data for transmittal to a local routertransmits the data in a path overhead field, thereby maintaining thedata in the state in which it was received from the remote router. Inone embodiment, the local router holding a protect interface uses thecopied data to configure a communications relationship with the localrouters, the ADM, and the remote router or routers.

In a system using ADMs that do not comply with the APS protocolrequirement to communicate with a protect interface using the APSprotocol, the system and method advantageously allows routers tofunction in a manner similar to that of a system using ADMs with APS orMSP. More specifically, once a router with a protect interface reads theidentifying signature data copied into path overhead bytes transmittedby a remote router, the router with the protect interface determineswhich local interface an ADM is reading. The router holding a protectinterface uses control logic to read the identifying signature data andto activate the correct interface.

As an example, referring to FIG. 3, according to an embodiment of amethod of the present invention, remote router 318 reflects anidentifying signature in frame path overhead bytes that indicates thatthe currently selected remote interface, either the working interface ofrouter 318 or the protect interface of router 320 received data fromrouter 314. The path overhead bytes are received by both the workinginterface of router 314 and the protect interface of router 316. Usingcontrol logic, router 316 reads the identifying signature data copiedinto path overhead bytes transmitted by a remote router 318 or router320. Next, using control logic, router 316 determines that the workinginterface in router 314 is the interface that ADM 310 is reading andactivates the working interface of router 314.

Another embodiment of the present invention concerns the signaling linkbetween a working and a protect interface. As discussed above, in eitheran APS or an MSP system, the protect interface and the ADM negotiate,using the APS protocol, whether the ADM should listen to the working orprotect interface. The protect interface activates either the working orprotect interface. The signaling link between the working and protectinterfaces provides the communication path whereby the protect interfacecontrols whether the working interface is or is not activated. Theproper functioning of APS/MSP systems therefore depends on continuouscommunication between the working and protect interfaces, using acommunication link independent of the protected circuits.

In general, when the communication link between the working and protectrouters fails, the working interface is activated and the protectinterface is deactivated. In situations in which the communication linkfails during a period in which the protect interface is active, thefailure of the communication link therefore causes an APS switch tooccur, and the protect interface requests, via the APS protocol, thatthe ADM select the working interface. If, at this time, the workinginterface, or the circuit connecting the working interface with the ADM,is a failed condition, then connectivity to remote routers is lost.Meanwhile, the protect interface, which did not fail and, therefore,could take over communications, disables itself.

A further problem concerning the signaling link between a protectinterface and a working interface occurs when the failure that causedlost communications is in the signaling link itself. In some cases, boththe protect and the working interface may send data to ADMsimultaneously, causing service disruptions.

According to an embodiment of the present invention, a signaling linkfailure does not default to a working interface. Instead, upon asignaling link failure, both a working interface router and a protectinterface router keep the same communication configuration. Morespecifically, both working and protect interfaces use the identifyingsignature data reflected from a remote router interface to determinewhich interface is currently being read by an ADM. Thus, each interfaceactivates or deactivates its own link to the ADM as needed.

More specifically, referring to FIG. 4, if a signaling link failureoccurs between the working interface of router 414 and the protectinterface of router 416, then router 416 uses the reflected informationtransmitted in the path overhead to determine the selection state of theADM, and activate or deactivate its own link accordingly. In oneembodiment, both router 418 and router 420 receive the identifyingsignature data in the path overhead, and both copy the signature databack in the outgoing path overhead for transmittal to router 414 androuter 416. By reading the copied signature data, both router 414 androuter 416 determine the selection state of ADM 410. If ADM 410 iscurrently selecting the link to the working interface of router 414,router 414 activates its link to the ADM and the protect interface ofrouter 416 deactivates its link to the ADM. Conversely, if ADM 410currently selects the link to the protect interface in router 416, thenthe working interface in router 414 deactivates its link to the ADM andthe protect interface in router 416 activates its link to the ADM. Inanother embodiment, router 416 uses a reflected path trace as well asany information received via the link between the protect interface inrouter 416 and the working interface in router 414 to automaticallyconfirm or supply the configuration data for the ADM. According to thisembodiment, interfaces are capable of verifying that a local interfaceis compatible with the remote router configuration requirements.

OTHER EMBODIMENTS

Although particular embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of the invention. For example, oneof skill in the art appreciates that the disclosed embodiments apply toPacket over SONET (POS) and other similar interfaces, but are notlimited to POS interfaces. For example, the embodiments relating to asystem and method for supporting ADMs that do not use switchingprotocols apply to POS type interfaces as well as Asynchronous TransferMode (ATM) type interfaces. For purposes of identifying the broaderaspects of the appended claims, word “a” is generally intended to mean“one or more.”

1. An apparatus for a communications network, the apparatus comprising: at least one interface circuit that reads frame data received from the communications network and writes frame data to be transmitted over the communications network, the frame data including a plurality of transport overhead fields; and signature logic coupled to the at least one interface circuit, wherein the signature logic identifies signature data and writes the signature data into at least one of a plurality of transport overhead fields in an outgoing frame, and the signature data identifies a type of the at least one interface circuit.
 2. The apparatus of claim 1 further comprising: reflector logic coupled to the at least one interface circuit, wherein the reflector logic copies data from at least one of the received transport overhead fields, the copied data being placed into a transport overhead field in the outgoing frame, the copied data including the received signature data.
 3. The apparatus of claim 1 wherein the type of the at least one interface circuit is one of a multiplex section protection (MSP) working circuit, a MSP protect circuit, and a non-MSP circuit.
 4. The apparatus of claim 1 wherein the identifying signature data includes data identifying the interface as one of an automatic protection switching (APS) working circuit, an APS protect circuit, and a non-APS circuit.
 5. The apparatus of claim 2 wherein the at least one interface circuit compares the copied data to earlier received frame data from the communications network to determine whether the copied data matches signature data identified in the earlier received frame data, the determination of a mismatch identifying a transition by a multiplexer.
 6. The apparatus of claim 5 wherein the transition is between a first one of a plurality of routers at a remote location and a second one of said routers.
 7. The apparatus of claim 5 wherein the transition is one of an APS switch and an MSP switch.
 8. The apparatus of claim 2 wherein the at least one interface circuit compares the copied data to later received frame data from the communications network to determine whether to update at least one routing table.
 9. The apparatus of claim 2 further comprising: another plurality of interface circuits disposed in at least one router, the router coupled via the communications network to the at least one interface circuit wherein the router reads the copied data including the signature data identifying one of the another plurality of interface circuits as an active interface, and wherein the router uses the copied data to configure a communications relationship.
 10. The apparatus of claim 9 wherein the at least one of the another plurality of interface circuits is associated with a protect interface, the protect interface being an active interface when transmission of data is disrupted to a working interface among the another plurality of interface circuits.
 11. The apparatus of claim 9 wherein the at least one of the another plurality of interface circuits includes a protect interface router and a working interface, the protect interface functioning as a backup interface, the working interface functioning as a primary interface, wherein at least one router housing the protect interface and the working interface uses the copied data to determine configuration compatibility between the protect interface and the working interface and to determine configuration compatibility among a plurality of tributary interfaces.
 12. The apparatus of claim 9 wherein the router uses the copied data to determine configuration compatibility among the another plurality of interface circuits and the at least one interface circuit.
 13. An apparatus for a communications network, the apparatus comprising: at least one interface circuit that reads frame data received from the communications network and writes frame data to be transmitted over the communications network, the frame data including a plurality of transport overhead fields; and signature logic coupled to the at least one interface circuit, wherein the signature logic identifies signature data and writes the signature data into at least one of a plurality of transport overhead fields in an outgoing frame, and the transport overhead field is a path level overhead field.
 14. The apparatus of claim 13 wherein the path level overhead field is a byte of a multi-byte path trace message conveyed by a path trace byte.
 15. The apparatus of claim 14 wherein the path trace byte is represented by a Synchronous Optical NETwork (SONET) path trace byte of a SONET OC-3c frame, according to a STS-3c standard for SONET, the path trace byte being designated by J1.
 16. The apparatus of claim 1 wherein the communications network includes a plurality of add-drop multiplexers, the plurality of add-drop multiplexers receiving and transmitting the copied data in one of a plurality of transport overhead fields while maintaining the copied data.
 17. An apparatus for a communications network, the apparatus comprising: at least one interface circuit that reads frame data received from the communications network and writes frame data to be transmitted over the communications network, the frame data including a plurality of transport overhead fields; and signature logic coupled to the at least one interface circuit, wherein the signature logic identifies signature data and writes the signature data into at least one of a plurality of transport overhead fields in an outgoing frame, and the communications network is a fiber optic network.
 18. An apparatus for a communications network, the apparatus comprising: at least one interface circuit that reads frame data received from the communications network and writes frame data to be transmitted over the communications network, the frame data including a plurality of transport overhead fields; and signature logic coupled to the at least one interface circuit, wherein the signature logic identifies signature data and writes the signature data into at least one of a plurality of transport overhead fields in an outgoing frame, and the communications network is one of a Synchronous Digital Hierarchy (SDH) and a Synchronous Optical NETwork (SONET).
 19. An apparatus for a communications network, the apparatus comprising: at least one interface circuit that reads frame data received from the communications network and writes frame data to be transmitted over the communications network, the frame data including a plurality of transport overhead fields; and signature logic coupled to the at least one interface circuit, wherein the signature logic identifies signature data and writes the signature data into at least one of a plurality of transport overhead fields in an outgoing frame, the signature logic is a program product, and the program product comprises signal bearing media comprising instructions for implementing: means for identifying the signature data, and means for writing the signature data into at least one of the plurality of transport overhead fields in an outgoing frame.
 20. The apparatus of claim 19 wherein the signal bearing media further comprises recordable media.
 21. The apparatus of claim 19 wherein the signal bearing media further comprises transmission media.
 22. The apparatus of claim 2 wherein the reflector logic is a program product and wherein the program product comprises signal bearing media comprising instructions for implementing: means for copying data from received transport overhead fields, and means for placing the copied data into a transport overhead field in an outgoing frame.
 23. The apparatus of claim 22 wherein the signal bearing media further comprises recordable media.
 24. The apparatus of claim 22 wherein the signal bearing media further comprises transmission media.
 25. A method for a communications network including at least one local router and at least one remote router, the method comprising: transmitting data in a transport overhead field to at least one remote router, the data identifying an active interface in the local router; receiving the data at the local router reflected from the remote router; and configuring a communications relationship using the data.
 26. The method of claim 25 further comprising: avoiding alteration of the data by one or more add-drop multiplexers.
 27. The method of claim 25 further comprising: in the remote router, using the data to determine which among a plurality of local interface circuits is the active interface in the local router.
 28. The method of claim 25 further comprising: in the remote router, using the data to determine whether there has been a transition among a plurality of local interface circuits, the transition changing the identity of the active interface in the local router.
 29. The method of claim 25 wherein the transport overhead field is a path level overhead field of a frame, the path level overhead field being received and transmitted by a plurality of intermediate add-drop multiplexers, the plurality of intermediate add-drop multiplexers maintaining the transport overhead field.
 30. The method of claim 29 wherein the path level overhead field is a byte of a multi-byte path trace message conveyed by a path trace byte.
 31. The method of claim 30 wherein the path trace byte is represented by a Synchronous Optical NETwork (SONET) path trace byte of a SONET OC-3c frame, according to a STS-3c standard for SONET, the path trace byte being designated by J1.
 32. The method of claim 25 further comprising: comparing the data to later received frame data from the communications network to determine whether to update at least one routing table.
 33. The method of claim 25 further comprising: using the data to determine configuration compatibility among a plurality of interface circuits.
 34. The method of claim 25 wherein the communications network is a fiber optic network.
 35. The method of claim 25 wherein the communications network is one of a Synchronous Digital Hierarchy (SDH) and a Synchronous Optical NETwork (SONET).
 36. A system for a communications network, the system comprising: means for transmitting data in a transport overhead field to at least one remote router, the data identifying an active interface in the local router; means for receiving the data at the local router reflected from the remote router; and means for configuring a communications relationship using the data.
 37. The system of claim 36 further comprising: means for avoiding alteration of the data by one or more add-drop multiplexers.
 38. The system of claim 36 further comprising: means, in the remote router, for using the data to determine which, among a plurality of local interfaces, is an active interface.
 39. The system of claim 36 further comprising: means for comparing the data to earlier received data from the communications network to determine whether to update at least one routing table.
 40. The system of claim 36, wherein the system is a program product and wherein the program product further comprises: signal bearing media comprising instructions for implementing: the means for transmitting data in the transport overhead field to the at least one remote router, the data identifying the active interface in the local router, the means for receiving the data at the local router reflected from the remote router, and the means for configuring the communications relationship using the data.
 41. The system of claim 40, wherein the signal bearing media further comprises recordable media.
 42. The system of claim 40, wherein the signal bearing media further comprises transmission media. 